1. Field of the Invention
This invention relates to a method of manufacturing a monolithic crystal filter, and more particularly to a method of manufacturing a monolithic crystal filter by plating its electrodes in sequence to establish desired values of resonator coupling and midband frequency in sequence.
2. Description of the Prior Art
The W. D. Beaver et al. U.S. Pat. No. 3,564,463 is directed to a monolithic crystal filter in which two identical pairs of electrodes (e.g., gold) are plated, such as by vapor deposition, onto opposite sides or faces of an AT-cut quartz crystal body or wafer in a process known in the art as mass-loading of the wafer. When one pair of the electrodes is connected to a source capable of exciting thickness shear mode vibrations in the wafer, and when the other pair of electrodes is connected to a resistive load, the electrode pairs form a pair of coupled resonators with the wafer. The coupling between the resonators, and the midband or center frequency of the filter, depend upon a number of factors, such as the degree of mass-loading, electrode separation, electrode dimensions and crystal wafer thickness. The Beaver et al. patent teaches that adjusting of the resonator coupling and the midband frequency can be accomplished by adding mass to the electrodes of the separate resonators to decrease their respective frequencies. However, since this change in mass-loading of the electrodes varies both the resonator coupling and the midband frequency simultaneously, fine control of the resonator coupling and midband frequency is difficult to achieve.
A more accurate procedure for establishing the coupling of the resonators and their midband frequency is disclosed in the I. E. Fair et al. U.S. Pat. No. 3,573,672. This procedure involves plating material in the gap between two of the electrodes on one side of the crystal wafer to increase or widen the coupling between the resonators. At the same time the midband frequency of the resonators decreases, but only by a small percentage of its value, whereby the resultant change is essentially insignificant. When the desired resonator coupling is reached, if the midband frequency is not within desired tolerance limits, additional mass is deposited onto the electrodes of the monolithic crystal filter to shift the midband frequency to the desired value. In this instance, the required shift in midband frequency represents only a small proportionate change in the midband frequency and the proportionate change in the value of the resonator coupling is insignificant.
More specifically, at the present time the manufacture of a monolithic crystal filter as disclosed in the Fair et al. patent involves base-plating the pairs of electrodes on opposite sides of the crystal wafer in apparatus of a known type. Simultaneously with the base-plating of the electrodes on one side of the crystal wafer, a resonator coupling-and-adjusting strip of the electrode material is plated in the gap between the electrodes in spaced relationship to the electrodes to produce a filter device having a resonator coupling greater than the resonator coupling desired. Additional electrode material then is vapor deposited on the electrodes in apparatus of the type disclosed in the E. C. Thompson U.S. Pat. No. 3,864,161, to produce a device having a midband frequency slightly above the desired midband frequency. Subsequently, a portion of the strip between the two electrodes is trimmed from the strip by a laser beam to decrease the resonator coupling to the desired value. In the alternative, additional electrode material may be added to the center strip to further increase the resonator coupling, if this is necessary. After the device has been attached or "floated" to a suitable metal header to complete the filter, additional electrode material is plated onto the electrodes of the filter if necessary, to fine-adjust the filter to the desired midband frequency.
The foregoing system has produced satisfactory results in the manufacture of monolithic crystal filters where the gap required between the split electrodes in order to achieve a desired width of resonator coupling can be greater than 30 mils, whereby the resonator coupling-and-adjusting strip can readily be formed between its associated electrodes in sufficiently spaced relationship with respect thereto. However, the system is not particularly suited for the fabrication of filters in which the gap between the electrodes must be narrowed in order to produce a resonator coupling of increased width. In this connection, in certain high frequency filters of a type having a pair of split electrodes on one side of the crystal wafer and a solid electrode on the opposite side of the crystal wafer, the gap or spacing between the pair of split electrodes may be so narrow (e.g., 5-30 mils) as to make it extremely difficult, and in some instances physically impossible, to fabricate the laser-trimmable resonator coupling-and-adjusting strip between the electrodes with known masking and plating techniques. Accordingly, the use of the above-described system is generally limited to filters having a midband frequency of less than 20 MHz and a resonator coupling less than 4 KHz. The use of the system also requires a significant investment in laser trimming equipment, which is not always practical from an economic standpoint. Accordingly, the purpose of this invention is to provide a new and improved method of fabricating monolithic crystal filters having a desired resonator coupling and midband frequency, without utilizing a laser-trimmable resonator coupling-and-adjusting strip between the pair of split electrodes.